1. Field of the Invention
The present invention relates to a television set having an active matrix type display apparatus.
2. Description of Related Art
A personal computer (PC) has widespread and the number of users who have a plurality of personal computers in homes increase. During such users, there is a request of connecting the personal computer to a television set. However, a wide area is necessary for installation of a plurality of monitors, and a fee is required for the number of monitors. Thus, a new demand arises in which only the personal computer is purchased without a display unit and the television set is used as a display unit. Typically, a display with a tuner is referred to as a television set, and a display with no tuner is referred to as a monitor.
As one of the typical thin type displays, an active matrix type liquid crystal display apparatus is known. In the liquid crystal display apparatus, scanning lines are used to select rows of pixels and data lines are supplied with display signals for grayscale data of the pixels. The pixel is arranged at each of intersections of the scanning lines and the data lines, and is provided with a TFT (Thin Film Transistor) transistor and a pixel electrode. Liquid crystal is filled between the pixel electrode and a common electrode opposite thereto. As the liquid crystal, a normally black liquid crystal is used in which a transmittance is the lowest (black) in a voltage non-application state. This normally black liquid crystal will be described below.
The liquid crystal display apparatus employs a method of inverting the polarity of the display signal supplied to the pixel, in order to prevent the liquid crystal material from being deteriorated. In other words, the pixel is alternately driven. As the inverting method, the following 4 methods are known:
[1] A frame inversion drive in which the polarity of the voltage applied to the common electrode is inverted for each frame, while the polarities of the display signals supplied to the data lines are not changed.
[2] A line inversion drive in which the polarity of the voltage applied to the common electrode is inverted for each horizontal synchronization and for each frame, while the polarities of the display signals supplied to the data lines are not changed.
[3] A column inversion drive in which the polarity of the display signal is inverted for each frame, such that the voltage of the common electrode is fixed and the polarities of the display signals of the data lines adjacent to each other are different, as shown in FIGS. 1A and 1B.[4] A 1H dot inversion drive in which the polarity of the display signal is inverted for each horizontal synchronization and for each frame, while the voltage of the common electrode is fixed and the polarities of the display signals of the data lines adjacent to each other are different, as shown in FIGS. 2A and 2B.
It is known that flicker is likely to be recognized in case of a same display pattern as a driving method. In a frame inversion drive, the flicker is likely to be recognized in a perfectly gray display. In a line inversion drive, the flicker is likely to be recognized in a horizontal stripe pattern. In a column inversion drive, the flicker is likely to be recognized in a vertical stripe pattern. In a dot inversion drive, the flicker is likely to be recognized in a checker pattern.
When the flicker, a crosstalk and the like are totally determined, the image qualities are degraded in the order of a 1H dot inversion drive, the line inversion drive, the column inversion drive and the frame inversion drive. This order also implies the order of larger electric power consumption. The image quality and the electric power consumption have the relation of trade-off.
In the liquid crystal television in which the number of the effective scanning lines is 720 or more (so-called high definition liquid crystal television), the increase in the number of the scanning lines causes one horizontal synchronization period to be short and also causes the number of pixels to be increased. Thus, the capacitance of the common electrode is increased, which prevents the voltage of the common electrode from being stabilized within a predetermined period. Therefore, it is difficult to employ a method of inverting the voltage of the common electrode (the frame inversion drive and the line inversion drive). In the following description, the merit and demerit of the column inversion drive and the 1 or 2H dot inversion drive in which the voltage of the common electrode is fixed will be described.
The merit of the column inversion drive lies in that the electric power consumption is small. The demerit lies in that the flicker and the vertical crosstalk are likely to be generated and the moving image quality is poor. As mentioned above, the flicker is likely to be recognized in the vertical stripe pattern. The vertical crosstalk is likely to be generated in the pattern in which white or black is displayed on a window portion as shown in FIG. 3 and its circumference is displayed in a middle gray-scale pattern.
The vertical crosstalk in the column inversion drive is mainly caused through change of the voltage of the pixel electrode in one frame period due to the off-current of the pixel. The off-current of the pixel is varied due to the voltage difference between the source and drain of the TFT. The same polarity of voltage is applied to the pixel, which is scanned in an initial part of the frame, in the majority part of the frame. Thus, the voltage difference between the source and drain of the TFT is small. The opposite polarity of voltage is applied to the pixel, which is scanned in the final part of the frame, in the majority part of one frame. Thus, the voltage difference between the source and drain of the TFT is large. In short, as the voltage difference between the source and drain of the TFT is larger, the off-current of the pixel is greater. In particular, the pixel scanned in the final part of the frame is large in a voltage variation amount. Accordingly, the flicker and the crosstalk are likely to be generated.
The merit of the 1H dot inversion drive lies in excellent image quality. Although the flicker is likely to be recognized in a checker pattern, the flicker and the crosstalk are small in the other patterns. The demerit lies in that the electric power consumption is large. Also, when the number of the scanning lines is increased, a brightness inclination is likely to be generated. The brightness inclination implies a phenomenon that a contrast is high at the pixels near to a data line driving IC is and that the contrast is low at the pixels distant from the data line driving IC. When a perfectly white pattern whose drive voltage is high is displayed, the pixels on a near-side are bright and the pixels on a distant side are dark. According to a drive waveform shown in FIG. 4, in the pixels on the distant side, the waveform of a display signal is dull, and the display signal cannot be sufficiently applied to the pixel electrode. This is because the data line becomes long in the larger scale of a liquid crystal panel, so as to increase parasitic capacitance of the data line. Also, the design of a higher definition increases the number of the scanning lines and makes one horizontal synchronization period short. Also, in the 1H dot inversion drive, the data lines adjacent to each other are temporarily shorted to collect charges before the polarity is switched, in order to make the electric power consumption small. By the charge collection, the electric power to charge and discharge the charges to and from the data line is reduced to ½. However, the charge collecting period is required, which reduces a write period to the pixels.
FIGS. 5A and 5B are diagrams showing the polarity of the pixel to which the 2H dot inversion drive is applied. The merit of the 2H dot inversion drive lies in that the electric power consumption is small, as compared with the 1H dot inversion drive. The demerit of the 2H dot inversion drive lies in that the waveform dullness of the display signal causes a horizontal pattern irregularity to be generated in a perfectly white pattern, a perfectly gray pattern and the like. FIG. 6 shows a waveform view of the 2H dot inversion drive. In FIG. 6, the waveform of the display signal on the distant side of the data line is represented by a solid line. According to this, although in an (m−1)th horizontal period, the waveform dullness is generated and a target voltage is not attained, an objective voltage is attained in an mth horizontal period. Also, in FIG. 6, the waveform of the display signal supplied to a (m−1)th pixel electrode is represented by an alternate long and short dash line, and the waveform of the display signal supplied to an mth pixel electrode is represented by an alternate long and two short dashes line. The waveform dullness causes the difference to be generated between the waveform in the (m−1)th pixel electrode and the waveform in the mth pixel electrode, and the voltages written into the pixel electrodes are made different, which results in the horizontal line irregularity.
The fact that the image quality of the dot inversion drive is good is known from Japanese Patent Application Publication (JP-P2001-042838A). This publication describes a technique for carrying out the dot inversion drive with no relation to a signal system used in a liquid crystal display apparatus. On the contrary, the fact that the image quality is poor when the polarity of the display signal is inverted for each frame or each field is also known from Japanese Patent Application Publication (JP-A-Heisei, 11-352938). This publication describes that a flicker is likely to be recognized when the polarity of a display signal is inverted at a time of shift from an odd-numbered field to an even-numbered field, in an interlace drive. Japanese Patent Application Publication (JP-P2000-180820A) describes a technique for carrying out a color display without using color filters. In this technique, one frame is divided into a red field, a green field and a blue field, and the polarity of the pixel electrode is inverted in each field.
In the column inversion drive, in order to decrease a voltage variation amount of the pixel electrode, the flicker and the vertical crosstalk can be reduced by carrying out at least one of the following items:
[a] making a drive frequency high,
[b] increasing a storage capacity of the pixel, and
[c] decreasing the off-current of the pixel.
In the column inversion drive, a technique for making the drive frequency high is known from Japanese Patent Application Publication (JP-P2002-091400A). This publication describes that a motion detecting circuit is provided in a video signal processor, and the motion detecting circuit carries out the column inversion drive by making a drive frequency high at the time of a video signal and carries out the dot inversion drive at a time of a still image.
In the technique described in the Japanese Patent Application Publication (JP-P2002-091400A), when the motion detecting circuit in the video signal processor automatically switches a drive system between the dot inversion drive and the column inversion drive, the flicker is reversely increased. At the time of the shift from the column inversion drive to the dot inversion drive, the drive frequency is delayed. In the first frame, the pixels driven in the same polarity are arranged every two columns. Thus, the flicker is horizontally generated. Moreover, in the display on the personal computer, since there are many moving images, which are cyclically flashed, the flicker becomes outstanding if the driving system is changed.
In short, in order to effectively operate the motion detecting circuit, it is necessary to adequately set a judgment reference for a motion in the moving image. For this purpose, the frame memories for several frames or more are required in the video signal processor. This increases the circuit scale of the video signal processor.
Also, in the dot inversion drive, when the drive frequency is made high in order to improve the image quality, the electric power consumption is increased. The increase in the electric power consumption results in heat generation in the data line driving IC and remarkable shortening in the life of the data line driving IC.